Method for generating photons by sonoluminescence

Abstract

A method of generating photons by sonoluminescence, from a gas bubble trapped in a liquid reservoir ( 2 ) by a standing ultrasound wave. An ultrasound impulse emitted by high-frequency transducers (T 1 –T 8 ) is superposed on the standing wave, the high-frequency transducers being pre-focused onto the gas bubble and pre-synchronized with the light emissions from the gas bubble during an initial training stage in which said focusing and said synchronization are optimized.

1. A method of generating photons by sonoluminescence, said method comprising at least the following steps:
 (a) generating at least one standing acoustic wave (S 1 ) in a liquid reservoir ( 2 ), said standing acoustic wave having at least one antinode; 
 (b) trapping at least one gas bubble ( 5 ) in the liquid at said antinode of the standing acoustic wave, said gas bubble then being subjected to a periodic deformation cycle comprising expansion stages ( 10 ) and contraction stages ( 12 ) in alternation; and 
 (e) generating acoustic wave impulses (S 2 ) in the liquid, which compression acoustic wave impulse are superposed on the standing acoustic wave (S 1 ), and cause photons to be emitted by the gas bubble, by sonoluminescence; 
 said method being characterized in that the acoustic wave impulses (S 2 ) are caused to be emitted by a number n at least equal to 2 of impulse firing transducers (T 1 –T 8 ) disposed around the gas bubble ( 5 ); 
 in that a focusing training step (c) and a synchronization step (d) 0 are interposed between the steps (b) and (e); and 
 in that, during the focusing training step (c), the impulse firing transducers are caused to emit acoustic wave impulses (S 2 ) with a first amplitude that is sufficiently small to avoid disturbing significantly the position and the deformation cycle of the gas bubble ( 5 ), acoustic signals generated by said acoustic wave impulse in the liquid reservoir are measured, and time offsets are deduced therefrom to be applied to respective (T 1 –T 8 ) so as to focus said acoustic wave impulses onto the gas bubble ( 5 ); 
 in that, during the step (d), instants at which acoustic wave impulses (S 2 ) are emitted by the various impulse firing transducers (T 1 –T 8 ) are determined so that each wave impulse ( 2 ) generated by the impulse firing transducers reaches the gas bubble ( 5 ) either during a contraction stage if the wave impulse is compression wave, or during an expansion stage if the wave impulse is an expansion wave; and 
 in that, during the step (e), the impulse firing transducers (T 1 –T 8 ) are caused to generate the acoustic wave impulses at the respective emit instants determined at the step (d), with a second amplitude that is large than the first amplitude. 
 
     
     
       2. A method according to  claim 1 , in which the step, (c) comprises the following sub-steps;
 (c1) each impulse firing transducer (T 1 –T 8 ) is caused to emit an acoustic wave impulse (S 2 ) in succession, with said first amplitude; 
 (c2) after each acoustic wave impulse emission, each impulse firing transducer  9 T 1 –T 8 ) is caused to measure the acoustic signals S 3   ij (t) generated by said acoustic wave impulse propagating in the liquid reservoir ( 2 ), and said measured signals are stored, I and J being indices respectively designating the impulse firing transducer that emitted the acoustic wave impulse and the impulse firing transducer that received the acoustic wave impulse corresponding to each measure signal s 3   ij (t); and 
 (c3) at least on the basis of said measured signals s 3   ij (t), said time offsets to be applied to respective ones of the acoustic wave impulses generated by the ones of the acoustic wave impulses generated by the various impulse firing transducers (T 1 –T 8 ) are determined so as to focus said acoustic wave impulses onto the gas bubble ( 5 ). 
 
     
     
       3. A method according to  claim 2 , in which, during the sub-step (c3), travel times taken by the acoustic wave impulses to travel between each impulse firing transducer (T 1 –T 8 ) and the gas bubble are determined, and said time offsets to be applied to respective ones of the acoustic waves generated by the various impulse firing transducers so as to focus said acoustic wave impulses onto the gas bubble are deduced from said travel times. 
     
     
       4. A method according to  claim 2 , in which a preliminary calibration step (a0) is performed, at least before the step (b), said calibration step comprising the following sub-steps:
 (a01) each impulse firing transducer (T 1 –T 8 ) is caused to emit an acoustic wave impulse (S 2 ) in succession, with said first amplitude; 
 (a02) after each acoustic wave impulse emission, each impulse firing transducer (T 1 –T 8 ) is caused to measure acoustic signals s 1   ij (t) generated by said acoustic wave impulse propagating in the liquid reservoir ( 2 ), and said measured signals s 1   ij (t) are stored; 
 
       during step (c) each impulse firing transducer (T 1 –T 8 ) is caused to listen to the acoustic signals s 2   j (t) received while the standing acoustic wave is being emitted in the presence of the gas bubble ( 5 ); 
       and during the sub-step (c 3 ), corrected signals s ij ( ( 1 )−s 3   ij (t)−s 1   ij (t)−s 2   j (t 0  are calculated, and then said time offsets are determined on the basis of the said corrected signals. 
     
     
       5. A method according to  claim 4 , in which said time offsets are determined by cross-correlation between said corrected signals. 
     
     
       6. A method according to  claim 1 , in which n is at least equal to 8. 
     
     
       7. A method according to  claim 1 , in which the acoustic wave impulses (S 20  are compression acoustic wave impulses, and, during the step (d), emit instants are determined at which the compression acoustic wave impulses (S 20  are emitted by the various impulse firing transducers (T 1 –T 8 ) so that each compression acoustic wave impulse (S 2 ) generated by the impulse firing transducers reaches the gas bubble ( 5 ) during a contraction stage. 
     
     
       8. A method according to  claim 7 , in which, during step (d), emission of the compression acoustic wave impulse by the various impulse firing transducers is synchronized with the deformation cycle followed by the gas bubble so that said compression acoustic wave impulses generate an increase in the pressure of the liquid surrounding the gas bubble at least until the end of the said contraction stage. 
     
     
       9. A method according to  claim 7 , in which, during step (d), emission of the compression acoustic wave impulses by the various impulse firing transducers (T 1 –T 8 ) is synchronized with the deformation cycle followed by the gas bubble, so that each compression acoustic wave impulse generated by the impulse firing transducers reaches the gas bubble ( 5 ) substantially when said gas bubble has its maximum diameter. 
     
     
       10. A method according to  claim 7 , in which the compression acoustic waves (S 2 ) generate acoustic vibration of the amplitude at least equal to 8 bars in the liquid in the vicinity of the gas bubble ( 5 ). 
     
     
       11. A method according to  claim 7 , in which, during step (e), the compression acoustic wave impulse (S 2 ) coming from each impulse firing transducer (T 1 –T 8 ) is caused to be preceded immediately by an expansion acoustic wave impulse (S 3 ) which is adapted to reach the gas bubble ( 5 ) during the expansion stage ( 10 ) preceding the contraction stage ( 12 ) during the which said gas bubble receives the compression acoustic wave impulses. 
     
     
       12. A method according to  claim 1 , in which the standing acoustic wave (S 1 ) is caused to be generated by at least two standing wave generation transducers (T′ 1 , T′ 2 ) distinct from the impulse firing transducers (T 1 –T 8 ). 
     
     
       13. A method according to  claim 1 , in which the standing acoustic wave (S 1 ) is an ultrasound wave of frequency lying in the range 20 kHz to 30 kHz and of amplitude in the vicinity of 1.3 bars.